This application is based upon and claims priority of Japanese Patent Application No. 2002-053533, filed on Feb. 28, 2002, the contents being incorporated herein by reference.
1. Field of the Invention
The present invention relates to mixed crystal layer growing method and device, and a semiconductor device, and, for example, it is suitable for use in the vapor phase epitaxial growth of a silicon-germanium mixed crystal layer doped with carbon and boron as a base layer of a heterojunction bipolar transistor.
2. Description of the Related Art
The formation of a high-concentration and thinned base layer is indispensable to obtain a higher-speed bipolar transistor used in an IC.
In recent years, attention is being given to a heterojunction bipolar transistor HBT in which a silicon-germanium (Si(1xe2x88x92x)Gex) mixed crystal layer is used as a base layer from the hope of its application to a silicon bipolar device, a superlattice device, and the like. To put such an HBT to practical use, a technique of vapor phase epitaxially growing the silicon-germanium mixed crystal layer on a silicon substrate is under study.
In the HBT in which the silicon-germanium mixed crystal layer is used as the base layer, the injection of carriers from an emitter to a base can be made larger compared with a homojunction bipolar transistor, whereby a ratio hFE of a collector current to a base current in a grounded-emitter DC operation can be secured without increasing base resistance and base transit distance.
Generally, in the fabrication of an npn-type HBT, the technology of forming a dope layer in situ by the use of a doping gas while the silicon-germanium mixed crystal layer is being formed by a CVD method is effective. Diborane gas (B2H6) is sometimes used as a boron (B) source gas for the growth of the silicon-germanium mixed crystal layer which becomes a p-type base layer.
Moreover, it is thought that in a heat process after the formation of the base layer, the doping of carbon (C) is effective in inhibiting an increase in base width (a reduction in concentration) accompanying TED (Transient Enhanced Diffusion) of boron.
However, when the silicon-germanium mixed crystal layer is doped with boron and carbon in addition, desired concentrations of these impurities in the layer can not be sometimes obtained for some factor. As a result, when carbon, for example, is not sufficiently doped into a desired region, the diffusion of boron occurs in the heat process after the formation of the base layer, which sometimes causes a problem that the base transit distance, concentration, and emitter/base junction position can not be controlled, or the like.
The present invention is made in view of the aforementioned problem, and its object is to enable the effective inhibition of the diffusion of an impurity such as boron by doping an impurity such as carbon into a desired region with high precision when a silicon-germanium mixed crystal layer is grown.
Mixed crystal layer growing method and device of the present invention target mixed crystal layer growing method and device for growing a silicon-germanium mixed crystal layer on a substrate placed in a reaction chamber by introducing source gases into the reaction chamber. When the silicon-germanium mixed layer is grown using a silicon source gas, a germanium source gas, a first impurity source gas such as boron, and a second impurity source gas such as carbon for inhibiting the diffusion of a first impurity in the mixed crystal layer, the concentration of a second impurity is equal to or higher than the concentration of the first impurity in a region at any given depth of the silicon-germanium mixed crystal layer.
Moreover, a semiconductor device of the present invention targets a semiconductor device having a silicon-germanium mixed crystal layer, for example, a semiconductor device in which a bipolar transistor formed with a base layer, an emitter layer, and a collector layer is constructed on a semiconductor substrate, and the silicon-germanium mixed crystal layer is provided as the base layer. Further, the silicon-germanium mixed crystal layer is doped with a first impurity such as boron and a second impurity such as carbon for inhibiting the diffusion of the first impurity, and the concentration of the second impurity is equal to or higher than the concentration of the first impurity in a region at any given depth of the silicon-germanium mixed crystal layer.